Here's a list of trees (probably not exhaustive) in local communities where one can recite the Berachah. According to the creator of the list, anytime in Nisan seems to be acceptable (see the Hebrew comments on the right). misaskim.org/bracha.php?page=1
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Seth JApr 5 '11 at 16:31

1 Answer
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The bracha can be made from the time the tree starts to bloom, regardless of the month (Mishna Berura quoting "Acharonim"). Lechatechita it should be said before the fruit grows.

The Mishna Berurah sides that, bediavad, if you did not see the blossoming and the fruit started to grow, you can rely on those who say you can make a bracha until the fruit is fully complete (minimally edible?). It sounds like if you saw the blossom and passed up the opportunity, you should not rely on this opinion. The Aruch haShulchan does not side with that qualification.